The present disclosure relates generally to integrated circuit device design, and more specifically, to device isolation structures having improved thermal conductivity.
Silicon Germanium (SiGe) Heterojunction bipolar devices have been widely used in high speed and high frequency applications. For a typical bipolar transistor, the collector junction that is located at the bottom of the sandwich layers has more current flow compared with the base in the center layer and the emitter in top layer. In advanced SiGe Heterojunction bipolar transistors (HBTs), as current density goes up, and devices are scaled down in geometry, thermal heating becomes severe. Trench isolation, using a mid-depth trench, is not adequate for transferring heat out. Typically, conventional oxide material is replaced with high thermal conductivity materials (e.g. poly-Si) over a thick liner for electrical isolation purpose. Although thermal conductivity can be improved therein, the thick liner (˜300 Å) restricts the heat dissipation efficiency. Moreover, the filled material increases current leakage risk. With continually smaller geometric scaling and increased supplied power, the thermal effect becomes more and more significant. As such, conventional isolation structures including deep trench (DT) and trench isolation (TI) are inadequate for transferring sufficient heat away from the HBT. A more efficient heat dissipation method is desired.